This is the med-lab chapter but with tougher (and more numerous) enemies, and the first enemy that cannot be killed by mundane (anything you carry) means -- the regenerating necromorph. Check your stock and if you need to buy an air tank for later, do so (although you can also try and upgrade your RIG and increase your air supply by 20 to 40 seconds).

Check through your weapons too. If you are not going for the one-gun trophy, you will want to use the linegun and start collecting the ample amount of ammunition for it.

The poison in chemistry lab.

There is a dangerous wall mounted necromorph in the bottom level of the room leading to Zero G Therapy. This monster is dangerous, since it has the ability to kill Isaac in one hit (regardless of life remaining). The pods it generates will dock your life quickly, and while the plasma cutter is good for the pods, the wall necro itself is vulnerable to stasis and the ripper.

The time you take to change weapons will let enemies get in their licks. Try instead a mine (secondary fire) from the linegun on the wall necro and the linegun's primary fire on the pods. Although not wholly effective as the stasis and ripper, not changing weapons will let you deal with the threats more quickly.

Head up the lift into the chem lab. Use stasis on the fast moving platform and head into chemical research. Make sure your stasis meter is full -- once you take the chemical compound, freeze/stasis the regenerator, shear its legs off and wait for Kendra to unlock the door (it is on a timer). Stasis the monster again if it breaks free and simply unlock the door and get out. Naturally, before you activate the regenerator, get the free power node (1 of 2) in the same room.

In the room with the wall necro, there will be some facehugging enemies. Stasis them and use the linegun's mine or plasma cut them all. If they get on you, they will take out a hefty chunk of life, even on normal. Return to the med-lab's security room.

Specimen in Intesive Care Unit (ICU).

Refill on stasis before you leave the zero G area. You will need the stasis and ammunition/medical kits to weather the regenerator's return in the large surgical theater. Again, you have to wait for Kendra to open the door. The added challenge here is that there are many more enemies other than the regenerator attacking you. While tempting to clean out all the enemies but the regenerator, you may wind up using more than you get back.

If you decide to fight, clear out the legs first with the plasma cutter or linegun. You definitely need to spend some stasis packs for this (and after). Stasis the regenerator so it is locked up for a while. Hold back the enemies (pick up any items while running) and exit the surgical theater near the broken battery when Kendra unlocks the door. There is a second wall necro near the ICU entrance. Use a stasis on the wall necro and shred it and its pods quickly.

Once you get the specimen, the air on the deck will decompress, leaving you with little time to get back to the security station. Use the air station at the end of the ICU hallway to out-last the enemies if you find going through them too tough. The linegun's mines are good for the swarming aliens, but keep in mind you have to deal with the fat aliens in the surgery. Use stasis and open the door. Here, fighting will only get you in trouble. Activate the air in the control booth and breathe easy.

Poison mixture and cryogenics.

Go back into the chem lab and mix the sample with the antigen. The door to cryogenics will open and it will be a "boss fight" of sorts. You cannot beat the regenerating alien by any means save environmental solutions, so lure it into the freezing chamber, shear off its legs, and stasis it.

Once the regenerator is frozen, sprint to the control room and flash freeze the regenerator in the room. It should lock up the monster and put it in cold storage. Don't miss the free power node (2 of 2) in the cryo control booth. With the poison readied, exit to the tram. Keep in mind that the flamethrower will be mostly useless in the next chapter, so buy (and upgrade) what's been working for you.